Ionic liquids are substances, which are made up only from ions and have a melting point of <100° C. or are, ideally, liquid at ambient temperature. They have been proposed for use in electrolytes for lithium and lithium-ion batteries, as they exhibit relatively favorable electrochemical stability and high ionic conductivity. Despite the potential advantages, ionic liquids have not been widely used as electrolytes for lithium and lithium ion batteries due to a number of significant disadvantages. Although lithium-ion cells using LiMnO2 and Li4Ti5O12 as electrode materials show satisfactory cycling behavior using ionic liquid as electrolyte solvent, this cell configuration suffers from the relatively small voltage of 2.5 V. In addition, the cell has low rate capability due to the high viscosity and poor wettability of the ionic liquid with electrode materials.
Moreover, early experiments to cycle lithium-ion batteries using carbonaceous negative electrode materials and ionic liquid-based electrolytes failed. Any ionic liquid sample tested was reduced at the low potential at which the intercalation of lithium into the graphite proceeds. It is believed that the reduction of the ionic liquids proceeds due to the formation of dimeric species. For commercial applications, lithium metal is, however, not advantageous. Due to the high reactivity of its surface, lithium is potentially hazardous, especially at elevated temperatures. Proposals to stabilize lithiated graphite electrodes for use in lithium-ion batteries include admixture of small amounts of highly active film forming additives. Such additives could protect against the continued reduction of the electrolyte itself at the surface of the low potential graphite. However, in most cases, the additives have issues associated with the poor solubility in ionic liquid electrolytes.